Carbonless copying systems are known which use a top sheet, upon which an impression is placed by writing, typing or the like, and a copy sheet upon which a copy of the impressions placed upon on the top sheet appears. In such carbonless copying systems, the back surface of the top sheet (i.e., the surface of the top sheet on the opposed side from the surface on which the impressions are placed) is coated with a layer of a color precursor composition which, when pressure is exerted upon the top sheet as the impressions are placed thereon, releases a color precursor, a material which, in the form in which it is originally present on the top sheet, is either colorless or at least not strongly colored. The front surface of the copy sheet (i.e., the surface facing the back surface of the top sheet) is coated with a layer of a color developing composition which, when it comes into contact with the color precursor released from the top sheet, develops a strong color, thereby resulting in a colored copy of the original impression being produced upon the copy sheet. (The term "colored" as used herein does not exclude black and white, the only requirement being that contact between the color precursor and the color developing composition produces a color which can be perceived by the human eye. In many prior carbonless copying systems, the color developed is a deep blue or black, which makes a highly legible impression upon the copy sheet, which is typically a conventional white paper. In theory, one might use a color precursor/color developer composition combination which produce a white copy on a black copy sheet, although there are obvious practical difficulties to such an approach.)
It will be seen that, since the only functional requirement placed upon the color precursor and the color developing composition is that the color precursor be capable of being released from the top sheet and of interacting with the color developing composition to produce a colored impression upon the copy sheet, in theory the terms "color precursor composition" and "color developing composition" are interchangeable. Nevertheless, these two distinct terms will be used herein in their conventional sense in the art, since the chemical nature of the two materials is usually very different. The color precursors are typically complex organic materials containing strings of conjugated carbon-carbon bonds and are capable of existing in both an essentially colorless and a strongly colored form, depending upon the pH or degree of ionization of the organic compound. These organic color precursors are present in the layer on the top sheet either as liquids or as liquid solutions of the organic compounds. On the other hand, the color developing compositions are typically Lewis acids, often in the form of clays, which are spread on the front surface of the copy sheet and remain on that surface throughout the copying operation. However, this invention is not restricted to any particular form of color precursor or color developing composition, provided that the color precursor and the color developing composition used fulfill the aforementioned functional requirements.
One problem with such carbonless copying systems is that, for ease of use, the top and copy sheets are usually releasably secured to one another during manufacture so that they lie flat against one another with the surfaces of the sheets coated with the color precursor and color developing compositions adjacent and facing each other. Frequently, a plurality of sets of top and copy sheets are releasably secured together to form a pad of alternating top and copy sheets. (As will be well known to those skilled in the arts, such carbonless copying systems can be arranged to produce multiple copies by placing a second layer of color precursor composition on the back surface of the first copy sheet and providing a second copy sheet bearing a layer of color developing composition on its front surface, and so on. When such multiple-copy systems are assembled into pads, the pads of course comprises multiple sets of sheets, each set comprising one top sheet together with the requisite number of copy sheets.) Once the sheets have been thus assembled into pairs or pads, they are inevitably subject to stress due to handling, transportation and the like before they receive the desired impression on their top sheets and produce the desired copies of these impressions. Unfortunately, the undesired stresses due to handling, transportation and the like tend to result in unwanted release of color precursor from the top sheets, thereby producing undesired spurious and messy markings on the copy sheets.
Efforts have been made to eliminate such spurious markings by micro-encapsulation of the color precursor, the intent being that the microcapsules will be sufficiently robust that they will not rupture as a result of the relatively low stresses placed upon the sheets during handling, transportation and the like but will rupture readily under the much higher stresses produced when impressions are made upon the top sheet by writing, typewriting etc. Techniques for the production of such microcapsules are described in U.S. Re-issue Pat. No. 24899 issued Nov. 29, 1960 to Green (re-issue of U.S. Pat. No. 2800458) and in U.S. Pats. No. 2800457 issued July 23, 1957 to Green et al, and 2800458 issued on the same date to Green alone.
Although encapsulation of the color precursor does reduce the production of spurious markings, it creates difficulties in spreading the microcapsules uniformly over the back surface of the top sheet. As a practical matter, the only way of securing a sufficiently uniform distribution of the microcapsules on the top sheet is to apply the microcapsules in a liquid dispersion. Since most of the conventional micro-encapsulation materials used are natural organic compounds, such as gum arabic or gelatin, the microcapsules tend to be hydrophilic and hence the microcapsules are normally formed into an aqueous dispersion. A film of this aqueous dispersion is placed on the top sheet and then dried to produce the requisite layer of microcapsules. However, the placing of the aqueous layer on a top sheet made of paper (the usual sheet material, for obvious economic reasons), followed by drying of the paper sheet frequently results in distortion of the paper by curling and wrinkling.
Experiments have been conducted using a non-aqueous dispersion to apply the microcapsules to the paper. It might appear that one useful dispersion medium for the microcapsules would be paraffin wax or some similar waxy material, since such a material can be melted at a temperature which will not destroy the microcapsules, a liquid dispersion of the microcapsules in the melted wax spread on the paper, and the paper cooled to produce a solid, coherent layout of the microcapsules/wax dispersion on the paper without the risk of the paper curling or wrinkling. Unfortunately, because of the hydrophilic nature of most micro-encapsulation materials used commercially, the capsules themselves are, as already noted, hydrophilic and hence will not disperse easily in the intensely hydrophobic wax. Accordingly, it is extremely difficult to produce a sufficiently uniform dispersion of the microcapsules in the wax, and hence a sufficiently uniform distribution of the microcapsules on the paper. Wax is also more colored than is desirable for use in a color precursor composition and affects the color of the markings produced on the copy sheets.
In experiments leading to the present invention, an attempt was made to replace the intensely hydrophobic paraffin wax with a more hydrophilic thermoplastic synthetic resin, more specifically a polyethylene glycol, in the hope that the hydrophilic capsules would disperse more easily in a relatively hydrophilic synthetic resin, and that this synthetic resin could be melted and spread on the paper in the same way as paraffin wax. Unfortunately, it was found that the commercially-available polyethylene glycol sold under the Registered Trademark PEG 8000 by Union Carbide Corporation gave rise to several problems because of its water solubility, and it is believed that, having regard to their water solubility, substantially similar difficulties will be encountered in attempting to apply most prior art polyalkylene glycols as carriers for micro-encapsulated color precursors in carbonless copying systems. Because of the water solubility of the synthetic resin, any form of dampness, for example humidity, rain or even perspiration on the hands of the users, has extremely deleterious effects upon the coated sheets, for example, smudging, runny ink, bleed through, washing off of the ink, etc. However, PEG 8000 has no residual color, an acceptable transition temperature and good ability to disperse hydrophilic microcapsules. Accordingly, there is a need for a modified synthetic resin which, when used as a carrier for color precursor microcapsules, will retain the advantages of PEG 8000 while overcoming the difficulties due to its excessive water solubility. This invention provides such a modified synthetic resin, a process for the preparation, a color precursor composition containing the modified synthetic resin, a coated sheet for use in a carbonless copying system and coated with the color precursor composition of the invention, and a process for producing such a coated sheet.